A typical wireless communication system, such as specified in the Third Generation Partnership Project (3GPP), transmits downlink communications from a base station to one or a plurality of wireless transmit/receive units (WTRUs). An uplink communication occurs when the WTRU transmits to the Base Station (BS).
In a direct sequence Code Division Multiple Access (CDMA) transmission system, data is modulated by spreading it into a wideband radio frequency signal using a spreading code sequence. A communication system assigns different spreading codes to each user enabling them to communicate using the same radio frequency band. Receivers operate by correlating or despreading the received signal with a known spreading code sequence.
A receiver may receive time offset copies of a transmitted communication signal known as multi-path fading. The signal energy is dispersed over time due to distinct multi-paths and scattering. If the receiver has some information about the channel profile, the receiver may estimate the communication signal by combining the multi-path copies of the signal to improve performance. For example, one such method gathers signal energy by assigning correlator branches to different paths and combining their outputs constructively.
In a CDMA system, a Rake receiver is conventionally used. As shown in FIG. 1, a Rake receiver 10 consists of a bank of “sub-receivers” 20A, 20B . . . 20N and a combiner 30. Each “sub-receiver” 20 constitutes a Rake finger, i.e., multipath, which includes a delay 25A, 25B . . . 25N, a despreader 35A, 35B . . . 35N, a complex weight generator 45A, 45B . . . 45N and a demodulator (or multiplier) 55A, 55B . . . 55N, where the complex weight generator 45 estimates the channel gain. The channel gain is a complex parameter representing the amplitude attenuation and phase rotation of the signal received via antenna 60 and the sub-receivers 20. The demodulator (or multiplier) 55 is essentially a multiplier that multiplies the output of the despreader 35 with the complex weight provided by complex weight generator 45, whereby the output of multiplier 55 is a phase-rotation-removed and amplitude-weighted despread signal. Therefore, the combiner 30 coherently (or in co-phase) combines all signals received from all of the “sub-receivers” 20.
The Rake receiver 10 has several “fingers,” one for each path. In each finger, a path delay with respect to some reference delay, such as the direct or the earliest received path, must be estimated and tracked throughout the transmission. Rake receivers are able to exploit multi-path propagation to benefit from path diversity of the transmitted signal. Using a plurality of paths, or rays, increases the signal power available to the receiver. Additionally, it provides protection against fading since several paths are unlikely to be subject to simultaneous deep fades. With suitable combining, this can improve the received signal-to-noise ratio, reduce fading and ease power control problems.
In conventional wireless communication systems, there is a significant frequency offset between the Node B and the WTRU due to imprecise oscillators used in the WTRU. This frequency offset, which translates into a phase shift over time, must be estimated and corrected in the WTRU or else a significant loss in performance will occur. There are several conventional algorithms used for differential detection to estimate the phase shift in a constant velocity WTRU. The algorithms assume that the phase shifts between any two adjacent pilot symbols are constant over the observation window. The benefits of a Rake receiver sometimes are reduced because of complex algorithms required to perform frequency offset estimation and CWG which are processor and memory intensive, and consume valuable system resources.
FIG. 2 shows three prior art simulated phase shift estimation algorithms 205, 210, 215, using the square root of the phase Mean Square Errors (MSEs) of three estimators at a Signal-to-Noise Ratio (SNR) of 0 dB.
A first prior art algorithm 205 supposes that rk,j is the jth despread pilot symbol at the kth slot. The phase shift (difference) θ between two adjacent pilot symbols can be estimated, {circumflex over (θ)}, by Equation 1 as follows:                               θ          ^                =                  angle          ⁢                      {                                          ∑                                  k                  =                  1                                                  N                  1                                            ⁢                                                          ⁢                                                ∑                                      j                    =                    1                                                                              N                      2                                        -                    1                                                  ⁢                                                                  ⁢                                                      r                                          k                      ,                                              j                        +                        1                                                                              ⁢                                      r                                          k                      ,                      j                                        *                                                                        }                                              Equation        ⁢                                  ⁢        1            where N1 is the number of slots used for the phase shift estimation, and N2 is the number of pilot symbols per slot used for the phase shift estimation.
A second prior art algorithm 210 estimates the phase difference of two pilot symbols that are separated by one symbol and divides it by two, and is expressed by Equation 2 as follows:                               θ          ^                =                              1            2                    ⁢          angle          ⁢                      {                                          ∑                                  k                  =                  1                                                  N                  1                                            ⁢                                                          ⁢                                                ∑                                      j                    =                    1                                                                              N                      2                                        -                    2                                                  ⁢                                                                  ⁢                                                      r                                          k                      ,                                              j                        +                        2                                                                              ⁢                                      r                                          k                      ,                      j                                        *                                                                        }                                              Equation        ⁢                                  ⁢        2            from the performance point of view, the larger the separation of two pilot symbols, the better the performance. But there is a limitation on the separation, which is the number of pilot symbols per slot. If the separation is too large, the system will not know how many phase rotations occurred, which will cause errors. Therefore the minimum number of pilot symbols is three per slot and two pilot symbols that are separated by more than one symbol cannot be used.
A third prior art algorithm 215 estimates the phase shift by using two pilot symbols separated by one slot. The phase shift over one slot estimation, {circumflex over (θ)}0, is shown by Equation 3 as follows:                                           θ            ^                    0                =                  angle          ⁢                      {                                                  ⁢                                          ∑                                  k                  =                  1                                                                      N                    1                                    -                  1                                            ⁢                                                          ⁢                                                ∑                                      j                    =                    1                                                        N                    2                                                  ⁢                                                      r                                                                  k                        +                        1                                            ,                      j                                                        ⁢                                      r                                          k                      ,                      j                                        *                                                                        }                                              Equation        ⁢                                  ⁢        3            where −180°<{circumflex over (θ)}0≦180°. Since the phase shift over one slot is in the range of −295°≦10*θ≦295°, there are ambiguities to estimating θ from {circumflex over (θ)}0. The values for {circumflex over (θ)}0 can be found in Table 1.       θ    ^    =      {                                                      1              10                        ⁢                                          θ                ^                            0                                                                                                        10                *                θ                                                    ≤                          180              ⁢              °                                                                                      1              10                        ⁢                          (                                                                    θ                    ^                                    0                                +                                  360                  ⁢                  °                                            )                                                                          0              >              0                        ,                                                            θ                  ^                                0                            <              0                                                                                      1              10                        ⁢                          (                                                                    θ                    ^                                    0                                -                                  360                  ⁢                  °                                            )                                                                          θ              <              0                        ,                                                            θ                  ^                                0                            >              0                                          
The sign of θ is assumed to be known, and thus there is no ambiguity of prior art algorithm 215. The number of pilot symbols per slot is equal to 3. It is found that prior art algorithm 205 is the least effective and prior art algorithm 215 performs best, as expected.
The above prior art algorithms each have at least one problem. Prior art algorithm 215 outperforms prior art algorithms 205 and 210, but has phase ambiguity problems and cannot be used. Prior art algorithms 205 and 210 introduce a high noise variance.
Therefore new algorithms are needed which have better performance than the prior art algorithms 205 and 210, and which do not have the phase ambiguities of prior art algorithm 215. In addition, it would be desirable to these new algorithms produce complex weights needed for Rake reception that are less processor and memory intensive.